Design and Fabrication of a Metallic MEMS Gripper UsingElectro-thermal V-shape and Modified U-shape Actuators

Jay J. Khazaai, H. Qu, M. Shillor, L. Smith, Ka. C. Cheok

Oakland University, Rochester, MI, USAjkhazaai@oakland.edu

ABSTRACT

This paper reports the design, fabrication, andcharacterization of a distinctive MEMS gripper electro-thermally driven jointly by a new metallic V-shape actuator(VSA) and a set of modified Guckel U-shape actuators(mUSA). The modification of the angle (90°± ) betweenthe hot and cold arms in the mUSA facilitates desiredunidirectional in-plane displacement and thus increases theopening of the gripper. A tip opening up to 173 m hasbeen measured within the operating voltage range. Thisunique configuration distinguishes this MEMS gripper fromothers in the capability of generation of larger tipdisplacement and greater holding force. The metallicstructures allow a low operating voltage and low overallpower consumption. MetalMUMPs is employed to fabricatethe device, in which electroplated nickel is used as thestructural material.

Keywords: MEMS, Micro Gripper, Micro Actuator,Electro-thermal, MetalMUMPs.

1 INTRODUCTION

MEMS grippers, when opening, require largemechanical driving force and displacement. The actuatorsused in driving the moving parts of the MEMS gripper shallgenerate a controllable and stable displacement with desiredforce in a guided direction. MEMS actuators responding toelectrical signal within their transduction systems mainlyfall into four classes [1] i.e., electrostatic, piezoelectric,electro-thermal, and electromagnetic. Electrostatic actuatorssuffer from less functional robustness and small range ofcontrollable displacement. Electromagnetic andpiezoelectric actuators have fabrication processcomplexities and material unavailability issues. The electro-thermal actuators have been more widely used due to thecompatibility with standard IC fabrication process andmaterials; the capability in generating relatively largeactuation displacement and force; and structural andfunctional robustness.

Previously, the authors have reported the design andfabrication of polysilicon switches consisting of similarV-shape and modified U-shape actuators [2-3]. Despite ofthe large displacement generated by the demonstratedpolysilicon VSA and mUSA compared to other structures[4-5], in general, the achieved displacement of thepolysilicon actuators may still be insufficient for MEMS

grippers where quite large traveling distance (tens of µm)and force (range of mN) are normally needed. This ismainly due to the relatively smaller thermal expansioncoefficient of polysilicon than many metals. In design of theMEMS gripper with large opening, we have usedelectroplated nickel as the structural material for its largerthermal expansion coefficient and thus possible largedesired displacement with limited device size.MetalMUMPs foundry service is used for the devicefabrication. Critical design requirements such as large in-plane controllable actuator displacement and force in aguided direction, faster (re)initialization of the mechanicalstructure for faster (de)actuation, reliable alignment of thegripper tips, and minimized out-of-plane and rotationaldisplacements of the structure are considered in the design.It should be noted that the unique VSA designed in thisdevice is versatile and can be widely used as mechanicaldrivers in a variety of MEMS/BioMEMS devices, such asmanipulators, switches, surgical instruments, and needles.

2 DESIGN OF GRIPPER ACTUATORS

Fig. 1 illustrates the structure of the designed metallicgripper in which a 4-arm VSA and two 2-arm mUSAs areemployed. In operation, the simultaneous driving of thethree actuators allows for superposition of forces generatedby the VSA and mUSAs respectively and thus a greatlyamplified desired tip displacement in x-direction. Fig. 2 (a)and (b) depict the mechanical structures of the metallicVSA and the mUSA used in the gripper shown in Fig. 1.The VSA provides the adequate mechanical pushing force,and the mUSA generates the guided in-plane opening andholding force at the gripper tips. As presented in [2-3],

Figure 1: SEM image of a fabricated metallic MEMSgripper with structural configuration illustrated.

Tips

4-arm VSA

2-armmUSA

2-armmUSA

y

x

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compared to a straight connection (in the structure knownas Guckel or U-shape actuator - USA), the uniqueness ofthe mUSA is the connection of the hot arms, denoted by“hi” (with i =1, 2, 3), to the central joint beam with an angleof 90°- . Based on simulations, an optimized angle of= 5 is selected. The non-perpendicular jointing not only

guarantees the desired in-plane directional displacement butalso increases the traveling distance and actuating force.This distinguishes the MEMS gripper from the others[6-9] where USAs are used in the gripping mechanism.

It should be noted that the symmetric VSA in Fig. 2(a)can be considered as the combination of two 4-arm mUSAsthat are uniquely integrated, or the combination of chevronbeam actuators with unequal length and mUSAs. Theflexure-cold arm labeled as f1 and C1 has two functions, i.e.to minimize the out-of-plane displacement of the chevronhot beams, and to reinitialize their displacement as a springwhen the structures are de-actuated. In our VSA, due to thesmall cross sectional expansions, the focus is on one-dimensional analysis in which only the length changes ofthe structural beams are considered. The in-planedisplacement of the actuator tip Y resulted from lengthexpansions of the actuator beams is analyzed and discussedin [2-3] with polysilicon structures. Yet, the analyticalmodels for the displacement and force Fv discussed in thesepapers are also valid for the nickel VSA in this MEMSgripper. Upon actuation of the VSA, the lengths ofclamped-clamped chevron beams (hi - hi, with i= 2, 3, 4)shown in Fig. 2(a) expand to =L+ L. Consequently, ay-direction displacement Y at the central beam isgenerated, i.e.

1 cos( ) sin( )( ) sin cos sin( )

2 2 2vsa

L L LY (1)

As shown in Fig. 1, the VSA displacement Y and forceFv are applied to the two mUSAs to further drive thegripper tip opening. The mechanism is discussed in thefollowing section.

3 GRIPPER OPERATION MECHANISM

Fig. 3 illustrates the 3D models of the electro-thermalMEMS gripper before and after actuation.

In Fig. 2 (VSA) and Fig. 3 (gripper), f1, f2, C1, C2, h2,h3, h4, h5, g34, g23 g12, j0, j1, g0 denote the flexure, cold,hot, and joint beams, respectively. Upon actuating thegripper drivers, the y-direction in-plane force Fv anddisplacement Yvsa generated by the VSA is transmitted tothe mUSA beams f2, C2, g0, and ultimately h5 (fixed-end)via the central T-beam j0 and j1, resulting in mechanicalbending forces at the f2-h5 arm. Fig. 4 illustrates the gripperopening (mUSA displacement) due to the pushing force ofthe VSA. Simultaneously, the mUSA is energized toprovide a thermal buckling force to bend the flexures f2 andh5 further. With joint forces from the VSA and the twomUSAs, this double actuation mechanism results in a largergripper opening and holding force in x-direction.

y

x

Fv

Tip g0

4-arm VSA

Two mUSAs

j0h5

C2f2

j1

(a) (b)Figure 3: 3-D model of the gripper; (a) prior toactuation, and (b) after actuation.

Fu

A1 (+)A2 (-)

Fv

Fu X

+ Y

Figure 2: (a) A 4-arm V-shape Actuator (VSA), (b) a2-arm Modified U-shape Actuator (mUSA).

(b)

(a)

Fu

Figure 4: mUSA displacement; MEMS gripper openingdue to mechanical pushing force of the VSA.

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4 MODELING AND SIMULATION

The analytical modeling and simulation usingMATLAB are conducted to optimize the design parametersand predict the behavior of the device. As shown inFig. 5(a), the entire MEMS gripper can be modeled as anetwork of electro-thermal elements representing the beamsin the gripper. A general lumped-element model ofindividual beams and consequently the entire device ispresented in Fig. 5(b), in which Re, RT, CT represents theelectrical resistance, thermal resistance and thermalcapacitance, respectively. In device modeling, the effect oftemperature coefficient of resistance (TCR) of the beams isincluded. At a fixed voltage, total electrical current andpower generated in the device is a direct function of thetotal resistance. The thickness of the electroplated nickelstructure is H= 20 µm. The length by width dimensionsL×W (µm2) of the gripper structural beams are as; f1= f2=250×10, C1= C2= 508×25, h2= 758×10, h3= 878×10, h4=h5= 998×10, g34= g23= 20×15, g12= 40×15, j0= 364×15,j1= 188×40, g0= 130×25. As presented in [2-3], thetemperature distribution along the beam can be obtained by;

1 2( ) (0.5 ) ( ) 300eT x P L x x (K), where k is thethermal conductivity of the nickel. From the lumped modelshown in Fig. 5, the average temperature within thestructural beams can be obtained by,

22 2

0 0

( ) ( ) ( (0) ( ) ) exp[ ( ) ]T T TR R

e T e T T T

R R VV VT t T tR V R V R C

(2)

where, the voltage change effect is 1 2 0.50(1 )T r T eV R R V .

The TCR of nickel, r, is experimentally measured as 0.004(K-1). As discussed in Section 3, the total gripper opening isattributed to both the y-displacement YVSA from the VSAand the x-displacement XmUSA from the two mUSAs. Theopening mUSAX due to the thermal buckling displacement ofthe mUSA can be obtained from,

1 55 5 5

5

cos( )( ) sin(cos ( )) sin( ) sin( )hmUSA h h h

h

LX L L (3)

where, h5, Lh5, Lh5 are the steady state, initial, and thechange of the hot arm h5 length, respectively. With theforce and displacement transfer mechanism illustrated inFig. 4, the x-direction opening resulted from the pushingforce from VSA can be expressed as,

12 2

2 2

sin[ ]( ) cos (1 )mUSA yVSAVSA

f C tipf C tip

XY (4)

Using initial geometrical parameters given in above, thebeam length change caused by the temperature change Tcan be obtained by (1 )TL T , where the thermalexpansion coefficient of nickel is T= 0.0064 (K-1).Ultimately, the total gripper opening at the tip can beobtained by mUSA mUSA yVSAX X X . The holding force ofthe gripper Fu can be derived based on the spring stiffness

of the mUSA and the total displacement of the gripper tip.CoventorWare, a finite element analysis (FEA) tool

dedicated for MEMS simulation has been used in designverification. Fig. 6 shows the simulated temperaturedistribution along the structural beams of the gripper. At1.0V driving voltage, the maximum temperature of ~470 Koccurs at the VSA cold arm C1. The gripper tip temperatureremains below 396 K. The average operating temperature is~385 K. Also, at this 1 V operating voltage, in steady state,the VSA tip displacement is Y 22 µm which results inthe gripper tip opening of X 163 µm with a gain of ~7.4.The out-of-plane z-displacement maintains less than0.01 µm, owning to the symmetric structural design. TheVSA pushing force Fv and the mUSA holding force Fu arepredicted as ~8.6 mN and ~4.9 mN, respectively. The FEAresults are in a good agreement with respective analyticalresults while both have shown reasonable predictions of theperformance of the fabricated gripper.

5 DEVICE TESTS

Tests on the fabricated gripper and the actuators haveshown attractive features. Fig. 7 shows a gripper in itsactuated state, and the displacement profiles as functions ofthe actuation voltage. As the driving voltage (Vcc=VA1(+)-VA1(-)) varies within 0.6~1.0 V, the loaded VSA

Figure 6: Temperature distribution within the beams of thegripper shown in Figures 1 and 3 at 1.0 V driving voltage.

Figure 5: MEMS gripper models; (a) network of electrothermal elements, (b) lumped-element model of thestructural beams or entire device.

(a) (b)

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parabolically generates in-plane displacements withinY 2.7-13.5 µm causing the gripper openings withinX 5.5-173.5 µm (gain: ~2-12.8). The gripper opening

sensitivity (dX/dV) alters within ~0.27-0.57 µm/mV.Although, the displacement response to temperature andvoltage changes is nonlinear, but within Vcc=0.6~1.0 V, theVSA displacement sensitivity (dY/dV) remains constant at~0.027 µm/mV indicating its controllability and linearityunder the mechanical load as voltage changes. From~3.6 µm buckling of a spring test structure, the VSApushing force Fv measured ~8.4 mN at Vcc=1.0 V. Thegripper average temperature is determined as 320-380 Kwithin operating voltage of 0.6-1.0 V with consumingpower of ~0.34-0.85 W. More data are provided in Table 1.

Items Micro Gripper Electro Thermo Mechanical Spec.Description 4-arm VSA 2-arm mUSA

L×W×H Size (µm3) 2100×140×20 1050×140×20R Resistance ( ) 1.0 1.5T Max. Temperature (K) 470 423

X-axis Max. Xdisplacement (µm) 0.002 127.3

(tip: 173.5)

Y-axis Max. Ydisplacement (µm) 13.5 16.25

(tip: 29.7)

Z-axis Max. Zdisplacement (µm) -0.006 0.013

Table 1: Major specifications of the metallic MEMESgripper at 1.0 V operating voltage.

6 FABRICATION PROCESS

MetalMUMPs foundry service is employed for thedevice fabrication. A standard MetalMUMPs process [10]is used to fabricate the device shown in Fig. 1. The cross-sectional view of the fabricated device is illustrated inFigure 8. Electroplated nickel is used as structural material.To minimize the Joule heat radiated to the substrate, a25 m trench is etched underneath the gripper.

7 CONCLUSIONS AND DISCUSSIONS

A metallic MEMS gripper has been successfullymodeled, simulated and fabricated. Relatively, at a lowoperating voltage and power, the gripper averagetemperature stays low, and the VSA generates a large in-plane y-displacement causing a large gripper tip opening Xwhile the entire structure maintains very small out-of-planez-displacements. Within the operating voltage, the gripperopening sensitivity (dX/dV) remains high and alterspredictable. The VSA and mUSA designed in this grippercan also find applications in other associated MEMSdevices.

REFERENCES[1] D. J. Bell, et al, Journal of Micromechanics and

Microengineering, Vol. 15, 2005, pp.153-164.[2] J. J. Khazaai, et al, 2010 IEEE Sensors Conference,

2010, pp. 1454.[3] J. J. Khazaai, et al, 2010 NSTI-Nanotech, Vol. 2,

2010, pp. 681-684.[4] C. Kung, et al, Journal of the Chinese Institute of

Engineers, Vol. 28, No. 1, 2005, pp. 123-130.[5] Q. Huang, et al, Journal of Micromechanics and

Microengineering, Vol. 9, 1999, pp. 64-70.[6] K. Kim, et al, Microsystem Technologies, Vol. 10,

2004, pp. 689-693.[7] J. V. Crosby, et al, the Proceedings of the COMSOL

Conference, 2009, Boston.[8] N. Chronis, et al, Journal of Microelectro

mechanical Systems, Vol. 14, No. 4, 2005, pp. 857.[9] T. C. Duc, et al, Journal of Microelectro

mechanical Systems, Vol. 17, No. 6, 2008, pp.1546.[10] MEMSCAP Inc., MetalMUMPs Design Hand

Book, Revision 2.0.

Figure 8: Cross-sectional view of the fabricated gripper.

(a)

(b)Figure 7: (a) Optical image of the micro gripper actuatedat 1.0 V that generates a 173.4 µm opening; and (b)displacement response to voltage change; gripperopening versus VSA y-direction traveling distance.

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